Molecular simulations of droplet coalescence in oil/water/surfactant systems.

We report a molecular simulation study of the mechanism by which droplets covered with a surfactant monolayer coalesce. We study a model system where the rate-limiting step in coalescence is the rupture of the surfactant film. Our simulations allow us to focus on the stages at the core of the coalescence process: the initial rupture of the two surfactant monolayers, the rearrangement of the surfactant molecules to form a channel connecting the two droplets, and the expansion of the radius of the resulting channel. For our numerical study, we made use of the dissipative particle dynamics method. We used a coarse-grained description of the oil, water, and surfactant molecules. The rupture of the surfactant film is a rare event on the molecular time scale. To enhance the sampling of the rupture of the surfactant film, we used forward flux sampling (FFS). FFS not only allows us to estimate coalescence rates, it also provides insight into the molecular structure and free energy of the “transition” state. For a...